A deep Herschel/PACS observation of CO(40-39) in NGC 1068: A search for the molecular torus

Journal Article · Thu Oct 01 04:00:00 EDT 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/811/2/74· OSTI ID:22882544

Janssen, A. W.; Bruderer, S.; Sturm, E.; Contursi, A.; Davies, R.; Poglitsch, A.; Genzel, R.; Graciá-Carpio, J.; Lutz, D.; Tacconi, L.; Burtscher, L. ^[1]; Hailey-Dunsheath, S. ^[2]; Fischer, J. ^[3]; González-Alfonso, E. ^[4]; Sternberg, A. ^[5]; Veilleux, S. ^[6]

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1. Max-Planck-Institute for Extraterrestrial Physics (MPE), Giessenbachstraße 1, D-85748 Garching (Germany)
2. California Institute of Technology, 301-17, 1200 E. California Blvd., Pasadena, CA 91125 (United States)
3. Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave. SW, Washington, DC 20375 (United States)
4. Universidad de Alcalá de Henares, E-28871 Alcalá de Henares, Madrid (Spain)
5. Tel Aviv University, Sackler School of Physics and Astronomy, Ramat Aviv 69978 (Israel)
6. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)

Emission from high-J CO lines in galaxies has long been proposed as a tracer of X-ray dominated regions (XDRs) produced by active galactic nuclei (AGNs). Of particular interest is the question of whether the obscuring torus, which is required by AGN unification models, can be observed via high-J CO cooling lines. Here we report on the analysis of a deep Herschel/PACS observation of an extremely high-J CO transition (40-39) in the Seyfert 2 galaxy NGC 1068. The line was not detected, with a derived 3σ upper limit of 2×10{sup −17} W m{sup −2}. We apply an XDR model in order to investigate whether the upper limit constrains the properties of a molecular torus in NGC 1068. The XDR model predicts the CO spectral line energy distributions for various gas densities and illuminating X-ray fluxes. In our model, the CO(40-39) upper limit is matched by gas with densities of ∼10{sup 6}−10{sup 7} cm{sup −3}, located at 1.6–5 pc from the AGN, with column densities of at least 10{sup 25} cm{sup −2}. At such high column densities, however, dust absorbs most of the CO(40-39) line emission at λ=65.69 μm. Therefore, even if NGC 1068 has a molecular torus that radiates in the CO(40-39) line, the dust can attenuate the line emission to below the PACS detection limit. The upper limit is thus consistent with the existence of a molecular torus in NGC 1068. In general, we expect that the CO(40-39) is observable in only a few AGN nuclei (if at all), because of the required high gas column density, and absorption by dust.

• OSTI ID: 22882544
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 811; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
CARBON MONOXIDE
COSMIC DUST
EMISSION
ENERGY SPECTRA
GALAXY NUCLEI
SENSITIVITY
SEYFERT GALAXIES